Mold parts that are excessively heated, like spure bushings and areas near the gates, must be cooled intensively. Rapid and even cooling is enhanced by the use of highly conductive metals, such as berylliumcopper. These metals are used to full advantage in places where it is impossible to place sufficient cooling channels. Copper transfer twice as much heat as carbon steel and four times as much heat as stainless steel. This does not mean copper mold will run 4 times faster than a stainless steel mold but they will run some thin walled parts significantly faster.
Design For Manufacturing (DFM) report, it's a bridge between product developer and mold manufacturer. It has been implemented in many manufacturing industries and proved to a sufficient way to improve efficiency. A comprehensive DFM report for mold making project would be the first step to success. As a mold maker
, the more potential problems you foresee, the less risks you had in the manufacturing process. Here are a summary for it.
Many kinds of raw material are frequently used, such as PC, PC+ABS, ABS, PS, HIPS, Nylon, Material with glass or fiber filled suitability and price different of them are different. More importantly, the shrinkage rate is different, which is key factor to measurement and tolerance precision.
Here are few topics that need to be brought up or answered.
- Ensure all necessary data are completed and authorized; usually there will be 3D drawing for 3D design and NC programming, 2d drawing must be available especially when there is precision requirement, mold factory need pay special attention to the tolerance specified. If any of the tolerance are no able to fulfilled, bring it up to customer in advance, otherwise it would be a disaster when you found it after mold is build.
- Check with customer about the resin that they are going to use for production. If it happened to be a kind of resin that is not available in local market, the mold factory needs to come a solution with customer where the resin will come from. In most occasions, customer will accept to provide material for trail if it's a very special resin. If the resin is special or you don't have much experience of it, ask opinions of the customer for the shrinkage rate.
- Discuss through with customers about the primary design by presenting parting line, ejector pins location, sliders, lifter, engraving, surface requirement etc. this would save a lot of time afterwards for official design approval. Sometimes the product design against the manufacturability, compromise needs to be made to achieve the best result.
- It's important to deeply understand the demands of the customer, it's a key factor to influence the mold project cost, let customer know what steel you are going to use and how many shorts it guarantees, make sure it fits the demands.
- Timeline need to be settled down during meeting.
There are many types of injection mold and tooling in using today, such as two-plate, three-plate and stack molds. Two and three- plate molds are more commonly used for heavy wall and non-packaging products. Both cold and hot-runner systems are used for two and three plate molds. All stack molds use a hot manifold to convey the melt to the cavities. Each mold component must be machined and finished to exact dimensions with very tight tolerances and must be heat-treated to be able to withstand very high injection and clamp pressures. Injection molds are the most expensive molds used in plastics processing with very long lead times, required for their design and fabrication. Every injection mold must be tested and debugged to prove-out the ejection system, cooling and/or heating system and operating components before it is placed in production.
A gate is a designed small opening to allow the melted plastics into the mold cavity, a successful gate design is determined by gate type, dimensions, location, it's deeply related with the materials been used, the type of mold plates, and economic factors. Gate design is one of the most important elements to influence injection mold quality and productivity.
Single gate has always been selected as a priority unless the distance of the mold flow is too big and beyond the limits of the injection molding pressure. Multiple gates always cannot avoid the problem of weld line where the flows meet from different gates. This weld line are not allowed sometimes specially when the injection molded parts have critical appearance or strength requirement, single gate also have good performance on packing, part strength, lower scrap rates, which gating way to employ will be determined by the structure, dimension appearance requirement of the molded parts.
Many years ago we molded golf clubs driving heads much thicker than 25mm. One solution (also used for a number of other thick walled products) was to mold an insert to leave say a minimum 3mm wall depending on strength needed. The insert was made of all the regrind sometimes contaminated material. Molded at quite a fast cycle as distortion was not usually important for the insert. An extra mould and an extra operation but often proved economical with reduction in overall cycle time and material costs.
The marbling effect or tortoise shell effect developed in the last century was typically achieved by adding 2 or more different colors of the same basic resin and processing through a plunger style machine versus a reciprocating screw machine as is commonly found today. The reason it worked was because that type of injection molding machine did not mix the materials very well and you would have streaks of the dissimilar colors showing on the surface of the product.
An injection mold is a precision tool and also must be strong enough to withstand thousands of high pressure molding cycles, so the material of the injection mold must be carefully; it's normally special and expensive steel, The way of manufacturing injection mold have been changed along with modern technology developing, main steps are CNC milling, drilling, EDM, wire-cutting, CAD/CAE/CAM technology utilized in a wide range make the lead-time of mold making shorter and shorter.
When considering the secondary mold function, ie that of a heat exchanger it is not achieving uniform temperature across the injection mould which is important. The amount of heat energy absorbed by the mould varies dependent on the flow of material through the cavity. Around feed points there will be maximum heat generated due to material flow and shear. At the end of fill the material temperature will have reduced and the surface of the injection mold will absorb less heat energy.
Physical restrictions which restrict size of cooling channels such as small inserts, deep sections, etc have to be considered in each case. Also don't forget that whatever size your cooling channels, when you come to connect them to supply and return pipes you will have a reduction in area, due to the need to fit connectors.